Method of producing regular arrays of nano-scale objects using nano-structured block-copolymeric materials

ABSTRACT

A method of forming a periodic array of nano-scale objects using a block copolymer, and nano-scale object arrays formed from the method are provided. The method for forming the arrays generally includes the steps of depositing a block copolymer of at least two blocks on a substrate to form an ordered meso-scale structured array of the polymer materials, forming catalytic metal dots based on the meso-scale structure, and growing nano-scale objects on the catalytic dots to form an ordered array of nano-scale objects.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional application of U.S. patent applicationSer. No. 10/356,299 filed Jan. 30, 2003, now U.S. Pat. No. 7,115,305,issued Oct. 3, 2006, which is based on U.S. application Ser. No.60/353,290, filed Feb. 1, 2002, the disclosure of which is incorporatedby reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

The U.S. Government has certain rights in this invention pursuant togrant No. NAS 101187-2.1, awarded by the National Aeronautics and SpaceAdministration, Office of Space Science.

FIELD OF THE INVENTION

The present invention is directed to a method of producing regulararrays of carbon nanotubes, and more particularly to methods ofproducing regular arrays of carbon nanotubes using self-assemblingblock-copolymeric materials.

BACKGROUND OF THE INVENTION

Carbon nanotubes (CNT) have potential uses in a wide array ofapplications. In particular, regularly spaced arrays of CNTs areexpected to have wide ranging applications, either as new and noveldevices or as substantial improvements over existing technology. Carbonnanotubes are electrically conductive, have high strength and stiffness,and exhibit electro-mechanical coupling similar to piezo-electricmaterials: mechanical deformations of a CNT can induce charge transferinto the nanotube, and injected charge can produce deformation ormechanical stress in the nanotube.

These properties combine to give CNTs their versatility in a wide rangeof uses. One example uses an array of nanotubes as a filter medium inelectrophoretic separations of biomolecules. Electrophoretic separationoccurs through the differential transport of polyelectrolytes (DNA,proteins, etc.) through a porous medium in the presence of an electricfield. The medium acts as a sieve, producing a size-dependent mobilityof the molecules. Current state-of-the-art biomolecular analyticalsystems use polymer gels as sieves. Such gels typically have a wide,random distribution of pore sizes, often extending into size ranges muchgreater than that of the molecules of interest (1-10 nm), which limitsthe efficiency of such gels in separations. In addition, polymer gelsare susceptible to radiation damage and thermal decomposition, limitingtheir utility in analysis in harsh environments of interest to NASA. Aregularly-spaced array of carbon nanotubes, with well-defined diametersand tube-tube separations in the range of 1-10 nm, is expected to bemuch more efficient at such separations, much more amenable tominiaturization, and much more tolerant of harsh environments owing tothe high stability of CNT. Arrays of CNT would thus advance thistechnology and make these methods useful in many new environments.

Another set of applications for CNT arrays takes advantage of theirmechanical and electrical properties. The high strength and stiffness ofthese structures makes free-standing CNT excellent candidates forhigh-Q, high-frequency, nano-scale oscillators. In addition, theelectromechanical coupling provides a means by which to either excitevibrations (by applying an RF signal), or to monitor themotions/vibrations of nanotubes by monitoring the current induced asthey mechanically oscillate. Arrays of CNT have thus been proposed foruse as nanometer-scale RF filters, RF signal detectors and analyzers,and spectrum analyzers for mechanical vibrations (the “electronic ear”).In addition, nanotubes can be functionalized with various chemicalsubstituents to make various molecules bind to them more or lessstrongly. A nanotube so functionalized would be expected to change itsresonant frequency upon the attachment of a target molecule, so thatarrays of functionalized nanotubes could also be used as nano-scalechemical sensors (the “electronic nose”).

Finally, the small size and high aspect ratio of CNTs suggests that theywill give off electrons by field emission at much lower voltages thanthe (much larger) metal field emitter tips currently in use. A regulararray of independently-addressable nanotubes could thus be used as alow-voltage, low-power array of field emitters, for field emitterdisplays.

However, all of these applications rely on the ability to produce arraysof nanotubes with regular, well-defined inter-tube spacings. Thenanotubes would also need to have their aspect ratios (diameter andwall-thickness vs. length) carefully controlled, to ensure that theystood upright on the substrate, rather than bending over. Many previousstudies have reported mats of CNT catalytically grown on smoothsubstrates. These nanotubes are produced by the decomposition ofcarbon-containing gases (e.g., C₂H₄, CO) over nanometer-sized particlesof catalytic metals (e.g., Fe, Ni). The nanotubes nucleate upon themetal particles and grow longer as carbon is produced by decompositionof the source gases upon the catalyst. In such studies, catalystparticles are produced by deposition of catalytic metals onto thesubstrate, either physically or chemically, so that the resultantparticles are randomly distributed on the surface and have a widedistribution of sizes. Since the size of growing nanotubes is determinedby the size of the catalyst particle from which it nucleates, thisprocess results in growth of nanotubes with a wide diameterdistribution, positioned at random upon the substrate surface.

To date, most demonstrations of nanotube growth from such supportedcatalysts have yielded densely-packed “fields” of nanotubes: neighboringnanotubes are typically in contact with one another, and the sizedistribution is wide and not well controlled. Clearly, such an “array”does not have the properties needed for the applications discussedabove: nanotubes in contact cannot move or be addressed independently,and a wide range of diameters implies a wide range of resonantfrequencies, making spectral analysis as described above impossible.

“Bulk” methods for depositing catalytic metals, such as physical orchemical vapor deposition, will result in a stochastic distribution ofcatalytic particle sizes and locations and thus is not suitable forgenerating ordered arrays. Lithographic methods, such asphotolithography or electron-beam lithography, could work in principle,however photolithography does not have the resolution to makenanometer-sized particles, while conventional e-beam lithography will ingeneral be too slow for large-scale throughput and production of devicesusing these CNT arrays.

However, recently a technique has been developed for producinggeometrically regular self-assembled nanotube arrays with excellentuniformity. This process is based upon the self-organizing formation ofhighly uniform pore arrays in anodized aluminum films. First, ananochannel alumina structure is formed by anodizing an aluminum filmunder conditions that lead to hexagonally-ordered arrays of narrowchannels with very high aspect ratios. The nanochannel alumina structureis then used as a template for the growth of nanotube arrays of carbonand other materials, including metals and some semiconductors. The fullprocess technique is disclosed in Appl. Phys. Lett., Vol. 75, pg 367(1999), and is incorporated herein by reference. Utilizing thistechnique the authors were able to grow nanotube arrays comprisinguniform carbon nanotubes with a diameter of 32 nm. Despite the promiseof this new technique, it is a complex procedure, and can provide arraysof CNT with only a narrow range of diameters, limiting its usefulness insome applications.

Accordingly, a need exists for improved method of producing, fromcatalytic metal particles supported on substrates, an array of nanotubesthat are regularly spaced (in particular, they must not be in contact),and which have the same diameter and length (or at least have a verynarrow distribution of sizes) in order that they have nearly identicalelectrical and mechanical properties.

SUMMARY OF THE INVENTION

The present invention is directed to a method for producing a uniformarray of nano-scale objects utilizing self-assembling block copolymermolecules.

In one particular embodiment, the invention utilizes particular blockcopolymers to form ordered meso-scale structures, which can then be usedto coordinate the formation of regular arrays of nanometer-sized dots ofcatalytic materials which can serve as nucleation points from whichcarbon nanotubes can be grown. In such an embodiment, the size andseparation of the dots of catalytic material will depend on the size ofthe polymer blocks.

In still another embodiment, the catalytic dots have dimensions of a fewtens of nanometers.

In yet another embodiment, the solution of block copolymer includescatalytic metal compounds designed to associate preferentially with oneof the blocks of the block copolymer such that upon formation of theblock copolymer meso-scale structure, the catalytic metal compounds forma regular array of regions with high metal content, that can then beused to grow the carbon nanotubes.

In still yet another embodiment, the mixture is spin coated onto asubstrate.

In still yet another embodiment, the carbon nanotubes are formed fromthe decomposition of carbon-containing gases, such as methane, ethylene,acetylene or carbon monoxide.

In still yet another embodiment, the block copolymer meso-scalestructure is removed by oxidation to leave only the regular array ofdots of catalytic material.

In still yet another embodiment, a mixture of amphiphilic blockcopolymers and metal compounds having an appropriate associated polarityand/or philicity are utilized. In one such embodiment, the blockcopolymer monomers used are polystyrene and polymethylmethacrylate, themetal compound is iron chloride, and the substrate is silicon.

In still yet another embodiment, the method of producing the carbonnanotubes involves selectively removing the nanometer-scale structuresmade from one of the at least two different blocks from the substrate tocreate a plurality of voids in the polymeric array prior to theapplication of the catalytic material such that the catalytic materialbonds within the voids to form the catalytic material array. In such anembodiment, the step of selectively removing may involve a removalmethod selected from the group consisting of a UV induced decomposition,reactive ion etching, acid-base reaction, and oxidation.

In still yet another embodiment, the method of producing the carbonnanotubes includes the step of etching an ordered orientation guiderecess array into the substrate, from which the orientation of thecarbon nanotubes may be controlled. In such a method, the step ofetching may involve at least one process selected from the groupconsisting of metal etching, reactive ion etching, and SiO₂ etching.

In still yet another embodiment, the invention is directed to aself-assembled nano-array formed according to the block copolymer methoddescribed above. In such an embodiment the nano-array may be utilized asfilter media in electrophoretic separations, as high-Q high-frequencynano-scale oscillators, nano-meter scale RF filters, RF signaldetectors, analyzers, and field emitter tips.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention will bebetter understood by reference to the following detailed descriptionwhen considered in conjunction with the accompanying drawings wherein:

FIG. 1 is a schematic view of an embodiment of a generic process forforming a nanotube array according to the invention.

FIG. 2 is a schematic view of a first embodiment of a process forforming a nanotube array according to the invention.

FIG. 3 is a schematic view of a second embodiment of a process forforming a nanotube array according to the invention.

FIG. 4 a is an AFM image of an embodiment of a step of the processaccording to the invention.

FIG. 4 b is an AFM image of an embodiment of a step of the processaccording to the invention.

FIG. 4 c is an AFM image of an embodiment of a step of the processaccording to the invention.

FIG. 5 a is a schematic of one embodiment of a carbon nanotube arrayaccording to the present invention.

FIG. 5 b is a schematic of a preferred embodiment of a carbon nanotubearray according to the present invention.

FIG. 6 is an SEM of an embodiment of a randomly oriented carbon nanotubearray.

FIG. 7 is a schematic for a third embodiment of a process for forming anano-array sieve according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to a method of forming a periodicarray of nano-scale objects, such as carbon nanotubes fromself-assembling block copolymeric materials.

A generic embodiment of the process for forming ordered carbon nanotubearrays according to the current invention is shown schematically inFIG. 1. Although the materials used and the exact steps may vary, asshown, there are three general steps in the method for formingnano-arrays according to the present invention: 1) depositing the blockcopolymeric material 10 and 12 on a substrate 14 to form an orderedmeso-scale structure array of the polymer materials; 2) formation of thecatalytic metal dots 16 based on the meso-scale structure; and 3) growthof the carbon nanotubes 18 on the catalytic dots to form an orderedarray 20 of carbon nanotubes.

The proposed method makes use of the fact that a certain class ofmolecules, called. block copolymers, can spontaneously self-assembleinto regular arrays of nanometer-scale structures with periodicallyvarying chemical properties.

Block copolymers are polymeric molecules consisting of two or moresections, or “blocks,” each of which consists of a controlled number ofmonomers of a given type linked together as in a normal polymermolecule. Specifically, certain combinations of block copolymermolecules will, under appropriate conditions, self-assemble into larger,“meso-scale” structures having characteristic dimensions of about 5 toabout 100 nanometers. Accordingly, such a polymeric materialspontaneously acquires regular structure on a length scale substantiallylarger than the individual monomer units yet still much smaller than amacroscopic scale.

Using the different polymer blocks of such nanometer-structuredmaterials as local binding agents, it is possible to arrange regulararrays of nanometer-sized dots of catalytic metals from which to growarrays of carbon nanotubes. For example, using the differential bindingproperties (such as that arising from the differing polarities) of thepolymer blocks with respect to various metal-containing catalyticprecursor compounds one can selectively place clusters of catalyticmetals in a regular, ordered array.

As briefly discussed above, the present invention relies on thenanoscale separation of BCP materials to form a nanoscale array.Nanoscale phase separation in BCP materials occurs because of thediffering affinities, or binding properties, of the various blocks. Thisseparation is driven by the same forces that drive macro-scale phaseseparations, such as the separation that occurs between oil and water.For example, blocks composed of monomers with polar (hydrophilic)chemical groups will “prefer”, or have an affinity, to be in contactwith each other because their energy is lowest in this arrangementbecause of their dipole interactions. Likewise, blocks composed ofnon-polar (hydrophobic) monomers will tend to associate with each otherbecause the total energy of the system is lowest in this configuration.

For example, a block copolymer comprising polystyrene (PS) andpolymethylmethacrylate (PMMA) blocks of appropriate lengths will, uponheating, undergo a nano-scale phase separation and arrange itself intoan arrayed structure consisting of nano-regions of PMMA separated fromeach other in a matrix of PS. In this system, the separation occursbecause the PMMA monomers tend to have large dipole moments arising fromthe highly polarized oxygen-atom-containing chemical groups present inthese monomers. Likewise, the PS monomers are composed only of hydrogenand carbon atoms and so have only very small dipoles.

A first exemplary embodiment of a process for forming the nano-array 20of the present invention comprising nanotube nano-features utilizing ablock copolymer (BCP) technique is shown schematically in FIG. 2. In afirst step, a substrate 22, such as, for example, silicon is spin-coatedwith a mixture of block copolymer molecules 24 and a metal-containingcatalytic precursor compound 26. Although any such block copolymercapable of self-assembling into larger order array meso-scale structuresmay be used, in the embodiment shown in FIG. 2, a block copolymer havingblocks of polystyrene (PS) and polymethylmethacrylate (PMMA) isutilized. Likewise, although any suitable metal-containing compoundhaving differential binding affinities for the polymer monomers may beused, in the embodiment shown in FIG. 2, FeCl₃ is utilized.

For example, a BCP formed of blocks of both PMMA and PS will have bothpolar and non-polar chemical groups. Such molecules are said to be“amphiphilic” because they can interact with other molecules of eitherphilicity. Such differences in affinity within BCP molecules areexploited in the current invention to dose selected regions of the BCPwith other molecules. Specifically, if ionic or strongly polar metalmolecules are mixed with an amphiphilic BCP molecule in solution, thesemolecules will tend to associate with and bind preferentially with thepolar regions of the resulting BCP material when the solvent is removed,because as discussed above the total energy of the system is minimizedif charges or dipoles are near each other and can interact strongly.Likewise, if non-polar molecules are co-dissolved with BCP compounds,these molecules will tend to bind to the non-polar areas of theresulting BCP material.

In a second step, the spin-coated mixture is then annealed such that themonomers self-assembled into a well-ordered array structure havingregions of high metal content 28 and zero metal content 30.

In step 3, the BCP matrix is removed, such as by an oxidation processsuch that only an array of high metal 28 and zero metal content 30regions remain. In such an embodiment the size and shape of the regionswill depend on the size of the associated binding sites of the BCPmaterial.

In step 4, this regular array of metal dots 28 are then used to producea regular array of carbon nanotubes 32. Specifically, the substrate withthe array of metal regions is inserted into a furnace or vacuum systemthrough which is flowed an appropriate carbon-containing gas at elevatedtemperature, such as, for example, methane, ethylene, acetylene, orcarbon monoxide gas mixed in argon, nitrogen and/or ammonia at atemperature between about 600 and 900° C. The metal regions serve asnucleation points from which the carbon gases nucleate and grow carbonnanotubes 32. Because the size, shape, orientation, and spacing of thenucleation points may be controlled through the size and number of BCPmonomers used, the size, shape, orientation, and spacing of the carbonnanotubes can also be controlled.

A specific example of this process can be described with relation to amixture of a BCP composed of blocks of PS and PMMA, and FeCl₃. In thisexample, an amphiphilic BCP (PS/PMMA) is mixed with an ionic metal salt(FeCl₃) in an amphiphilic solvent, such as, for example, acetone. Thissolution is spin-coated onto a substrate, and then heated so that theBCP form pillars of PMMA in a PS matrix. Under such conditions, the Fe³⁺ions (and Cl⁻ ions) would migrate to the PMMA regions during heatingbecause the abundance of local dipoles in these regions (arising fromthe oxygen-bearing moieties) tend to attract and bind ionic species. Asa result, a regular array of iron-rich and iron free regions withnanometer dimension are formed, which when the polymer is oxidized away,such as by heating to 400° C. in air, leaves an array of Fe particlesfrom which a regular array of carbon nanotubes may be grown.

A second exemplary process for forming the nano-array 40 of the presentinvention comprising nanotube nano-features utilizing a block copolymer(BCP) technique is shown schematically in FIG. 3. In a first step, asubstrate 42, such as, for example, silicon is coated with a blockcopolymer only. Although any block copolymer capable of self-assemblinginto a meso-scale structured array may be used, in the embodiment shownin FIG. 2, a BCP consisting of polystyrene (PS) andpolymethylmethacrylate (PMMA) is utilized.

The spin-coated polymer layer is then annealed such that the BCPmolecules undergo nanoscale phase separation and arrange into an arrayedstructure consisting of cylinders 46 of one monomer separated from eachother in a matrix 48 of the second monomer. Using the exemplarymaterials, annealing would, for example, produce cylinders of PMMAsurrounded by a matrix of PS. As in the first exemplary process, thesize and separation of the PMMA cylinders would depend on the size ofthe PS and PMMA blocks in the molecules.

In step 2, the block cylinders 46 are removed, such as by an oxidationprocess. For example, in a PMMA/PS system, this would be done by firstexposing the thin film to UV light or high energy electrons, whichdegrades the PMMA, then washing in acetic acid to remove the degradedPMMA. Although only UV induced process is described above, any syntheticmethod that takes advantage of this type of differential reactivitybetween polymer blocks could in principle be used to fabricatenano-scale arrays from block copolymer materials. Additional examples ofsuch potential “differential chemistry” include differential reactivitywith respect to reactive ion etching (RIE), acid-base chemistry, andoxidation. Any such scheme could be exploited to preferentially removeone polymer and leave the other present as a template for nano-scalemetal deposition. In such an embodiment the size and shape of theregions will depend on the size of the associated binding sites of theBCP material.

As shown in step 3, this process leaves a thin film of polystyrene 48containing a regular array of holes 50, such that when metals 52 aredeposited either electrochemically or by vapor deposition onto thesurface of the substrate some of the metal is deposited onto the surfaceof the remaining polymer and some of the metal is deposited into theholes. As in the above embodiment, any suitable metal may be used, suchas Co, Ni, Mo, W, Fe, Pd, other transition metals, and alloys thereof.For example, in the embodiment shown in FIG. 3, Fe is utilized.

In step 4, the polystyrene matrix 48 is removed (e.g., by salvation)leaving an array of metal dots 52 upon the substrate 42.

In step 5, as in the first exemplary process, this regular array ofmetal dots 52 are then used to produce a regular array of carbonnanotubes 54. Specifically, the substrate with the array of metalregions is inserted into a furnace or vacuum system through which isflowed an appropriate carbon-containing gas at elevated temperature,such as, for example, acetylene gas diluted in nitrogen and/or ammoniaat about 650° C. The metal regions serve as nucleation points from whichthe carbon gases nucleate and grow carbon nanotubes 32. Because thesize, shape, orientation, and spacing of the nucleation points may becontrolled through the size and number of BCP monomers used, the size,shape, orientation, and spacing of the carbon nanotubes can also becontrolled.

One example of such technology uses a block copolymer denoted P(S-b-MMA)consisting of a length, or “block,” of polystyrene, typically 100 to1000 styrene monomers in length, attached to a block of PMMA, typically50-200 methylmethacrylate monomers long. Using a system comprisingP(S-b-MMA) composed of a PS block approximately 300 monomers longconnected to a PMMA block about 100 monomers long, producedself-assembled arrays of 14-nm diameter PMMA cylinders hexagonallypacked in a PS matrix with a lattice constant of 24 nm.

Specifically, in one embodiment toluene solutions of this P(S-b-MMA) arespin-cast onto substrates, forming films ˜1 micron thick. Thenano-structured array spontaneously assemble when the sample is heatedto 165° C. (well above the glass transition temperature) for 14 hours inan electric field of 40 V/Cm. Exposure to deep UV light followed byacetic acid rinse will remove the PMMA cylinders and leave a nanoporousPS film with 14 nm pores. These pores are then filled with cobalt metalby electrodeposition, yielding an array of cobalt “nanowires.” Theremaining polystyrene matrix is then removed using one of many availabletechniques including chemical treatment, solvation, oxidation, or RIE.Again, although this scheme utilizes the differential reactivity of thePS and PMMA blocks with respect to UV light/chemical treatment totransfer the nanoscale pattern within the polymer matrix to anunderlying substrate, in principle, any block copolymer that formsnano-domains in this way can act as such a template, if the variousblocks exhibit differential reactivity under some conditions which canbe used to remove one block without affecting the other. For example,differential reactivity with respect to Reactive Ion Etching (RIE) hasbeen used to transfer the nano-structured pattern in apolystyrene/polybutadiene (PS/PB) block copolymer matrix onto a siliconnitride substrate. PS/PB block copolymer forms nano-domains in a mannersimilar to the P(S-b-MMA) described above, and PB is etched much morerapidly than PS in a CF₄ plasma. Thus, on a substrate coated with PS/PBand processed with CF₄ RIE, areas under PB nanostructures can be exposedwhile those under the surrounding PS matrix remained covered.

Removal of the PS leaves behind a regular array of metal dots withdiameters determined by the pore size and heights determined by eitherfilm thickness or total flux of incident metal during deposition. Suchmetal particles are known to nucleate and grow nanotubes underappropriate conditions. Specifically, the substrates with metal dotarrays can be inserted into a furnace or vacuum system, through which isflowed carbon-containing gas molecules, such as C₂H₄ or CO. Upon heatingto appropriate temperatures (600-900° C.), the carbon-containingprecursor gas molecules will decompose on the catalytic metal surfaceswith minimal decomposition on other, non-catalytic regions of thesubstrate. The carbon generated at each metal particle will arrange intoconcentric graphitic layers and form a hollow tube, which grows awayfrom the catalyst particle as more carbon is generated by incoming gasmolecules.

FIGS. 4 a to 4 c show AFM micrographs of the various stages of producinga self-assembled array, including the formation of the meso-scale BCPstructure (4 a), the selective removal of one block of the BCP (4 b),and the deposition of the metal dots (4 c). As shown, because the porediameter is dependent on the size of the monomer block that is removed,the pore size can be controlled from a few nanometers up to severalhundred nanometers with densities as high as 10¹¹ cm⁻².

Although two exemplary methods of producing the ordered carbonnanoarrays are described above, as shown in FIG. 5 a, these methods bothdescribed processes in which the catalyst nucleation points 70 aredeposited on the surface of the substrate 72. As shown, under certaincircumstances, despite the ordered deposition of the catalyst nucleationpoints 70, the lack of an orientation for the catalyst nucleation pointsmay allow the growth of the carbon nanotubes 74 to occur in randomorientations. An SEM of a carbon nanotube array produced by aconventional growth method without the benefit of an orientation guideis shown in FIG. 6. However, as shown in FIG. 5 b, the growth andorientation of the carbon nanotubes 74 can be controlled by providing aninitial growth direction, such as by recessing the catalyst nucleationpoint 70 into the surface of the substrate 72.

A detailed diagram of one exemplary process for providing suchorientation control is shown in FIG. 7. As shown in steps 1 and 2, theprocess still entails the deposition and ordering of a BCP film 80 on asubstrate 82 and removal of one of the two polymer components 83 and 84to form well-ordered deposition spots 86. However, as shown in steps 2to 6, once the substrate 82 is exposed, further etching is conducted toprovide orientation recesses 88. Such etching may entail metal etching,and polymer and/or SiO₂ RIE. Once orientation recesses are etched in thesubstrate the metal catalyst 90 can be sputter or vacuum deposited ontothe substrate, as shown in step 7. Finally, as shown in step 8, once themetal catalyst is deposited, the remaining polymer component 84 isremoved by any conventional means leaving only the metal catalystdeposited within the orientation recesses 88.

In any of the above described embodiments, the size of the PMMA regions,and therefore the size of the ultimate carbon nanotubes depends on thesize of the PS and PMMA block in the BCP molecules. Likewise, in orderto form carbon nanotube arrays for a wide variety of applications, it isimportant to be able to control the spacing of the nanotubes on thesubstrate. Both of these objectives can be achieved by varying thelength of the different blocks in the block copolymer starting material.Specifically, the length of the blocks determines the amount ofmaterial, and hence the size, of the features in the nano-structuredpolymeric material arising from each block. In general, the size of thenanotube-nucleating metal particles can be determined by controlling thelength of the BCP block associated with the catalytic metal, while theirseparation can be controlled by varying the length of the other block.BCP's can be synthesized with a wide range of lengths of the two polymerblocks, making available a similar range of nanotube diameters andseparations in the resulting nanotube arrays.

For example, in the PS/PMMA material discussed above, the diameter ofthe PMMA cylinders varies as the length of the PMMA block is increasedor decreased, while the “size” of the PS matrix walls, i.e., theseparation between the PMMA cylinders, is determined by the length ofthe PS block. This exemplary embodiment, as discussed above, uses apolystyrene polymethylmethacrylate block copolymer denoted P(S-b-MMA)consisting of a length, or “block”, of polystyrene, typically 100 to1000 styrene monomers in length, attached to a block of PMMA, typically50-200 methylmethacrylate monomers long. Such a range of PS and PMMAblocks generally result in PMMA cylinders having diameters from about 10to about 100 nm hexagonally packed in a PS matrix with a lattice spacingconstant of about 10 to 100 nm. Again, as discussed above, in a recentdemonstration using this system, P(S-b-MMA) composed of a PS blockapproximately 300 monomers long connected to a PMMA block about 100monomers long, was found to self-assemble into arrays of PMMA cylindershaving diameters of 14 nm hexagonally packed in a PS matrix with alattice constant of 24 nm. However, P(S-b-MMA) can be easily synthesizedwith a wide range of lengths of the two polymer blocks, making availablea similar range of nanotube diameters and separations in the resultingnanotube arrays.

For some applications it is also important to be able to control thelocation of limited area nanotube arrays. With the present process thiscan be accomplished by using photomasking to control the area of UVexposure of the block copolymer film, or by using electron beamlithography to control which regions are exposed.

The invention is also directed to a nano-array formed from one of theprocesses discussed above, and although in the embodiments shown inFIGS. 1 to 3, the array of nanotubes comprises periodic rows of uniformnanotubes attached to the substrate, any other arrangement of nanotubescan be utilized such as, for example, staggered arrays of uniformnano-features or uniform rows of nanotubes having alternating sizes. Inanother alternative embodiment the spacing of the nanotubes may also bevaried.

Additionally, while FIGS. 1 to 3 only show an array deposited on a flatsubstrate surface, any geometry of substrate suitable could be utilized,such as, for example, a curved, corrugated or tubular substrate. It willbe understood that the design of the array according to the presentinvention is necessarily driven by the purpose of the array, and issensitively dependent on both the geometrical size, shape, orientationand spacing of the nanotubes as well as the size and properties of thearray itself. For example, the array itself has two characteristicsizes: (1) the nano-feature size, or pore size and (2) the nano-featurespacing. Accordingly, although the self-assembled nano-array of theembodiment shown in FIGS. 1 to 3 comprise a uniform array of uniformcylindrical nanotubes, it should be understood that any shape, size,orientation, or spacing of nanotubes, as described above, can beutilized in the nano-array of the current invention such that thephysical properties of the array are suitable for use in a device ofinterest.

Moreover, the nano-structured matrix into which the block copolymerassembles need not have the particular cylinders-in-matrix geometrydescribed above in order to be useful. A much more common motif for suchmaterials is for one block to form nm-size spheres, regularly arrangedin a 3-dimensional crystalline array within a matrix of the other block.Such structures can also be used as templates for nano-scale patterns onsubstrates provided that the polymer material can be deposited in a thinenough film so that only one or a few “layers” of the spherical domainsare present in the thickness of the film. Indeed, the PS/PB blockcopolymer referred to above is one such material, with the PB blocksforming spherical domains within the PS matrix.

In turn, while the self-assembled nano-arrays contemplated in many ofthe embodiments discussed in the present application are constructed ofcarbon nanotubes made from pyrolizing a carbon-containing feedstock overa substrate having an ordered array of germination points, thenanofeatures can be of any shape and made by any process and from anymaterial suitable for making self-assembled nano-features, such as, forexample, spheres or pyramids made of other atomic materials or evenbiomolecules, such as, for example, proteins. In another embodiment, thenano-features can also be further functionalized for a variety ofapplications, such as, for example, being made hydrophilic orhydrophobic, being charged either negatively or positively, or beingderivatized with specific chemical groups, etc.

Likewise, the substrate can be made of any material which can withstandthe temperatures required for growth of the nanotubes, and which can bemodified to provide a suitable ordered array of germination points forgrowing the nanotubes of the array, such as, for example, silicon,silicon dioxide, metals, alumina, glass, or even polymeric plastics.

Any suitable catalyzing metal can be used to activate the germinationpoints on the surface of the substrate, such as, for example, iron,nickel or cobalt. Alternatively, the catalyzing metal could be an alloyof two or more metals such as a Co/Ni alloy. The metal catalysts couldalso be produced by pyrolysis of inorganic or organic metal-containingcompounds, such as ferric chloride, ferric nitrate or cobalt chloride.Moreover, although FeCl₃, is repeatedly used as an exemplary metalmaterial, both hydrophilic and hydrophobic metal compounds abound.Examples of the former include all ionic salts of metals, such aschlorides, nitrates, etc. Examples of hydrophobic metals-containingcompounds include organometallic compounds such as metal carbonyls,metallocenes and acetylacetonates (propanedionates), complexes betweenthe metal atoms and cyclopentadiene (C₅H₆) and 2,4-pentanedione(CH₃CHOCH₂CHOCH₃), respectively.

Finally, although only “carbon” containing nanotubes are discussedabove, the general method may also be used to make nano-structured metalor semiconductor-containing materials. For example, in one embodiment anamphiphilic BCP consisting of blocks of polyethylene oxide (—CH₂—CH₂—O—,a hydrophilic monomer) and blocks of polypropylene oxide(—CH₂—CH(CH₃)—O—, a hydrophobic monomer) may be used to createnano-porous membranes of ceramics including SiO₂, Al₂O₃, TiO₂. In suchan embodiment, the metal-containing precursors (chloride salts) may beco-dissolved in alcohol with the BCP such that when the solvent isevaporated, the metal ions associate primarily with the more polarethylene oxide blocks, which form a matrix through which run cylindersof metal-free polypropylene oxide blocks. This arrangement is thenpreserved as the material is heated in air to oxidize away the BCP,resulting in a matrix of metal oxide through which runs a regular arrayof nanometer-scale pores

In another embodiment, the nanofeatures may also be semiconductingnanowires, such as solid nanometer-scale wires made of silicon. Thesesilicon nanometer-scale wires are of similar dimensions as the nanotubesand are produced very similarly. For example, in one embodiment siliconnanowires may be produced by placing the array of catalytic metal dotsproduced by any of the methods described herein in a reaction vesselcontaining a reactive silicon containing gas such as silane (SiH₄) ,rather than methane or ethylene. Although the same method may be used toproduce the ordered array of catalytic metal dots, the specificcatalytic materials may be different. For example, for a siliconnanowire, the catalyst is typically gold (Au), rather than iron.

Although specific embodiments are disclosed herein, it is expected thatpersons skilled in the art can and will design alternative methods toproduce the carbon nanotube arrays that are within the scope of thefollowing claims either literally or under the Doctrine of Equivalents.

Specifically, there are many different chemical conditions under whichone block of a copolymer may exhibit differing reactivity from otherblocks. Examples include acid-base chemistry, oxidation/reductionchemistry, and solvation. There also exist a large number of blockcopolymers known to form nano-structured matrices similar to thosedescribed herein. Thus a number of possibilities will arise for oneskilled in the art for using the self assembling nano-scale structure ofblock copolymers as a template to create regular arrays of catalyticmetal dots on substrate surfaces, and hence to grow regular arrays ofnanofeatures for a wide variety of applications.

1. A method of forming an ordered array of nanofeatures comprising thesteps of: depositing a block copolymer on a substrate, wherein the blockcopolymer comprises at least two different blocks, and wherein the blockcopolymer can self-assemble into an ordered polymeric array of aplurality of regular nanometer-scale structures; activating theblock-copolymer to self-assemble to form the polymeric array;selectively removing the nanometer-scale structures made from one of theat least two different blocks from the substrate to create a pluralityof voids in the polymeric array; applying a catalytic material such thatthe catalytic material bonds within the voids to form a catalyticmaterial array, wherein the catalytic material is chosen such that anordered catalytic material array is formed comprising a plurality ofcatalytic nucleation regions and having a structure defined by thepolymeric array; removing the polymeric array from the substrate suchthat only the catalytic material array remains; and growing nanofeatureson the catalytic material array to form an ordered nanofeature arrayhaving a structure defined by the catalytic material array.
 2. Themethod described in claim 1, wherein the step of selectively removingcomprises a removal method selected from the group consisting of UVinduced decomposition, solvation, reactive ion etching, acid-basereaction, and oxidation.
 3. The method described in claim 1, whereineach of the at least two different blocks is made of at least onepolymer monomer, and wherein the size, shape, and spacing of thenanometer-scale structures depends on the size and number of the polymermonomers in each of the at least two different blocks.
 4. The methoddescribed in claim 1, wherein the size of the catalytic nucleationregions depends on the length of the block which is selectively removedto form the voids into which the catalytic material bonds.
 5. The methoddescribed in claim 1, wherein spacing between the catalytic nucleationregions depends on a length of the block which is not selectivelyremoved in the formation of the voids into which the catalytic materialbonds.
 6. The method described in claim 1, wherein the at least twodifferent blocks self-assemble based on dipole interactions.
 7. Themethod described in claim 1, wherein the block copolymer is anamphiphilic block copolymer.
 8. The method described in claim 1, whereinthe step of depositing comprises spin-coating a solution containing theblock copolymer onto the substrate.
 9. The method described in claim 1,wherein the step of activating includes annealing the polymer coatedsubstrate.
 10. The method described in claim 1, wherein the step ofapplying comprises a method selected from one of vapor deposition,sputter deposition, and electrochemical application.
 11. The methoddescribed in claim 1, wherein the step of removing is selected from thegroup consisting of reactive ion etching, oxidation, and solvation. 12.The method described in claim 1, wherein the step of growing thenanofeatures further comprises heating the substrate bearing thecatalytic material array under a flow of a carbon-containing gas capableof reacting on the catalytic nucleation regions to form thenanofeatures.
 13. The method described in claim 12, wherein the step ofheating takes place in the presence of an additional gas selected fromthe group consisting of argon, nitrogen, and ammonia.
 14. The methoddescribed in claim 12, wherein the step of heating takes place in avacuum chamber at sub-atmospheric pressure.
 15. The method described inclaim 12, wherein the substrate is heated at a temperature of about 600to about 900° C.
 16. The method described in claim 12, wherein thecarbon-containing gas is selected from the group consisting of methane,ethylene, acetylene, and carbon monoxide.
 17. The method described inclaim 1, further comprising restricting the size of the nanofeaturearray by a method selected from one of photolitohography, photomaskingand electron beam lithography.
 18. The method described in claim 1,wherein the substrate is a complex shape selected from the groupconsisting of curve, corrugated and tubular.
 19. The method described inclaim 1, wherein the nanometer-scale structures are selected from one ofcylinders and spheres.
 20. The method described in claim 1, wherein thenanofeatures are a shape selected from the group consisting of tubes,spheres, pyramids and rectangles.
 21. The method described in claim 1,wherein the nanofeatures are carbon nanotubes.
 22. The method describedin claim 21, wherein the carbon nanotubes have a uniform diameter. 23.The method described in claim 1, wherein at least two differentlydimensioned nanofeatures are grown on the substrate.
 24. The methoddescribed in claim 1, wherein the nanofeatures have a cross-sectionaldimension of about 10 to 100 nm.
 25. The method described in claim 1,wherein the nanofeature array has a lattice spacing of about 10 to 100nm.
 26. The method described in claim 1, wherein the nanofeatures arefunctionalized.
 27. The method described in claim 1, wherein thenanofeatures are made from a material selected from the group consistingof metal oxides, silicon, or silicon dioxide.
 28. The method describedin claim 1, wherein the substrate is selected from the group consistingof metals, silicon dioxide, silicon, alumina, glass, and polymericplastic.
 29. The method described in claim 1, wherein the catalyticmaterial is selected from the group consisting of Mo, W, Pd, Fe, Ni, Coand a Ni/Co alloy.
 30. The method described in claim 1, wherein thecatalytic material is applied as an ionic metal salt.
 31. The methoddescribed in claim 30, wherein the ionic metal salt is selected from thegroup consisting of iron chloride, iron nitrate, nickel chloride, nickelnitrate, cobalt chloride, and cobalt nitrate.
 32. The method describedin claim 1, wherein the catalytic material is applied as anorganometallic compound.
 33. The method described in claim 32, whereinthe organometallic compound is selected from the group consisting ofmetal carbonyls, metallocenes, and acetylacetonates.
 34. The methoddescribed in claim 1, wherein the at least two different blocks arepolystyrene and polymethylmethacrylate.
 35. The method described inclaim 34, wherein the polystyrene block has a length of about 100 to1000 styrene monomers.
 36. The method described in claim 34, wherein thepolymethylmethacrylate block has a length of about 50 to 200methylmethacrylate monomers.
 37. The method described in claim 34,wherein the nanometer-scale structures comprise polymethylmethacrylatecylinders surrounded by a matrix of polystyrene.
 38. The methoddescribed in claim 1, wherein the nanofeature array is a uniformperiodic array having a uniform lattice spacing between nanofeatures.39. The method described in claim 1, wherein the nanofeature array hasat least two different lattice spacings.
 40. The method described inclaim 1, further comprising the step of etching an ordered orientationguide recess array comprising a plurality of orientation guide recessesinto the substrate prior to applying the catalytic material, such thatthe structure of the orientation guide array is determined by thepolymeric array.
 41. The method described in claim 40, wherein the stepof etching an ordered orientation guide recess array comprises at leastone process selected from the group consisting of metal etching,reactive ion etching, and SiO₂ etching.